1. Field of the Invention
The present invention relates to optical communication devices, and more particularly, to an optical communication device for detecting defects in optical fibers.
2. Description of the Related Art
Some optical communication devices employ a redundant configuration so that data transmission/reception can be performed even in the event of a fault occurring in the device (e.g., Unexamined Japanese Patent Publication No. H06-216847).
FIG. 5 shows an exemplary system configuration of conventional optical communication devices. As shown in the figure, an optical communication device 150 includes optical transmitters 151a, 151b, . . . , 151n, optical fibers 154aa, 154ba, 154ab, 154bb, . . . , 154an, 154bn, redundancy controllers 155a, 155b, . . . , 155n, redundant multiplexers 156a, 156b, . . . , 156n, an optical multiplexer 157, and an optical output unit 158. Another optical communication device 160 includes an optical input unit 161, an optical demultiplexer 162, redundant demultiplexers 163a, 163b, . . . , 163n, and optical receivers 164a, 164b, 164n. The optical communication devices 150 and 160 are connected by a WDM (Wavelength Division Multiplexing) transmission line 171.
The optical transmitter 151a includes an active unit 152 and a standby unit 153, each for converting an electrical signal to an optical signal. Specifically, the active and standby units 152 and 153 have transmitters 152a and 153a, respectively, both capable of converting an electrical signal to an optical signal and input with the same electrical signal. The transmitters 152a and 153a are configured in the same manner and convert the input electrical signals to optical signals of an identical wavelength.
The active and standby units 152 and 153 also have output controllers 152b and 153b, respectively. The output controllers 152b and 153b operate under the control of the redundancy controller 155a so that one of the optical signals output from the transmitters 152a and 153a may be supplied to the redundant multiplexer 156a. 
Usually, the redundancy controller 155a controls the output controllers 152b and 153b in such a manner that the optical signal output from the transmitter 152a is delivered to the redundant multiplexer 156a. In the event a fault or the like occurs in the transmitter 152a, the redundancy controller 155a controls the output controllers 152b and 153b so that the optical signal output from the transmitter 153a may be supplied to the redundant multiplexer 156a. 
The optical transmitters 151b, . . . , 151n are configured in the same manner as the optical transmitter 151a and each include active and standby units. The optical transmitters 151a, 151b, . . . , 151n are, however, adapted to convert electrical signals to optical signals of respective different wavelengths. The redundancy controllers 155b, . . . , 155n function in the same way as the redundancy controller 155a. Specifically, each of the redundancy controllers 155b, . . . , 155n usually controls its associated output controllers so that the optical signal from the active unit may be output and, in the event a fault or the like occurs in the active unit, controls the output controllers so that the optical signal from the standby unit may be output.
The redundant multiplexer 156a includes a coupler 156aa and outputs the optical signal supplied from the active or standby unit 152 or 153 to the optical multiplexer 157. Similarly, the redundant multiplexers 156b, . . . , 156n each include a coupler and output the optical signal supplied from the active or standby unit to the optical multiplexer 157. The optical transmitters 151a, 151b, . . . , 151n are connected to their corresponding redundant multiplexers 156a, 156b, . . . , 156n by the optical fibers 154aa, 154ba, 154ab, 154bb, . . . , 154an, 154bn. 
The optical multiplexer 157 multiplexes the optical signals output from the redundant multiplexers 156a, 156b, . . . , 156n and outputs the multiplexed signal to the optical output unit 158. The optical multiplexer 157 comprises optical filters, for example, and multiplexes the optical signals of different wavelengths, output from the redundant multiplexers 156a, 156b, 156n. 
The optical output unit 158 amplifies the optical signal output from the optical multiplexer 157 and outputs the amplified signal to the WDM transmission line 171.
The optical input unit 161 of the optical communication device 160 receives the optical signal from the WDM transmission line 171, amplifies the received signal, and outputs the amplified signal to the optical demultiplexer 162.
The optical demultiplexer 162 comprises optical filters, for example, and demultiplexes the wavelength-multiplexed optical signal output from the optical input unit 161 into optical signals of respective wavelengths. The optical signals of different wavelengths, demultiplexed by the optical demultiplexer 162, are output to the respective redundant demultiplexers 163a, 163b, . . . , 163n. 
The redundant demultiplexer 163a, which includes a coupler 163aa, separates the optical signal output from the optical demultiplexer 162 into signals of an identical wavelength and outputs the separated optical signals to the optical receiver 164a. Similarly, each of the redundant demultiplexers 163b, . . . , 163n includes a coupler and separates the corresponding optical signal output from the optical demultiplexer 162 into signals of an identical wavelength.
The optical receiver 164a includes an active unit 165 and a standby unit 166, each for converting an optical signal to an electrical signal. Specifically, the active and standby units 165 and 166 have receivers 165a and 166a, respectively, which are capable of converting an optical signal to an electrical signal and input with the identical optical signals. The receivers 165a and 166a are configured in the same manner and output electrical signals, obtained by converting the input optical signals, to circuits of the subsequent stage.
Let it be assumed here that the optical fiber 154ba connecting the standby unit 153 and the redundant multiplexer 156a is defective due to improper connection or fault and also that the control is switched to the standby unit 153 because of a fault in the active unit 152. In this case, since the standby optical fiber 154ba connecting the standby unit 153 and the redundant multiplexer 156a is defective, a problem arises in that signal loss is caused.